A porous ceramic article is widely used as a filter, catalyst carrier, and the like. More particularly, a porous ceramic article is used as an exhaust gas converter for a heat engine such as an internal combustion engine or for a combustion apparatus such as a boiler, a reformer of liquid fuel or gaseous fuel, a purification system for water or sewage, and the like. Many porous ceramic articles used in the above applications have a honeycomb structure to ensure a large processing area. These porous ceramic articles are generally obtained by adding a processing aid such as a pore-forming agent or a binder to a ceramic powder, kneading the powder mixture, forming the mixture in a prescribed form, and firing the formed product. The pore-forming agent is generally used to increase the number of pores and to control the size and volume of pores in the porous ceramic article.
Various materials functioning as the pore-forming agent have been proposed. For example, Japanese Patent Application Laid-open No. 55-100269 proposes a method for producing a cordierite honeycomb structure using starch powder. The Application discloses a method for producing a cordierite honeycomb structure, comprising mixing 100 parts by weight of a ceramic raw material to become cordierite by firing, 1–30 parts by weight of starch powder, a binder, and water, kneading the mixture, forming the kneaded product by extrusion molding, and drying and firing the formed product. The Application describes that starch powder is added to produce pores by firing. Specifically, starch powder leaves vacant spaces (pores) after incineration during firing. The numerous pores thus formed allow a large amount of catalyst to adhere, ensure a sufficient catalytic effect, and remarkably increase thermal shock resistance of the honeycomb structure.
As described in the Application, starch is excellent as a processing aid to form pores. However, use of starch causes deformation, or defects in shape in many cases, of the ceramic green body in the drying step after forming. Particularly, it may be difficult to obtain sufficient dimensional accuracy for a complicated form such as a honeycomb structure in which many cells are formed from intersecting thin walls. Therefore, to produce a porous ceramic article excelling in dimensional accuracy, a method of suppressing deformation in the drying step has been desired.
The porosity of a porous ceramic honeycomb structure used as an instrument for exhaust gas purification tends to further increase, since a decrease in pressure loss is requested. Therefore, the amount of pore-forming agent such as starch tends to increase. However, since an increase in the amount of pore-forming agent added is accompanied by an increase in calorific value during combustion of the pore-forming agent in the firing step, cracks may be easily formed in the ceramic article. This leads to difficulty in obtaining a nondefective porous ceramic article. In recent years, a foamed resin having a small calorific value has been preferably used as a pore-forming agent to produce a porous ceramic article having a high porosity. However, if the foamed resin alone is added as the pore-forming agent for a ceramic raw material having high hardness such as alumina or silicon carbide, a part of or a considerable amount of the foamed resin is crushed by the ceramic raw material during kneading. This causes a problem such as a decreased or nonuniform pore-forming effect. Therefore, it may be difficult to obtain a porous ceramic article having a desired porosity. Accordingly, a method for producing a porous ceramic article having a high porosity without producing cracks or the like in the firing step has been desired.